Storage systems are systems that are used to mange and to store files. A storage system usually comprises several storage devices, whereof some are high speed storage devices such as hard disc drives and whereof others are slower storage devices such as tape drives. In principle, it is preferable to keep all files on high speed storage devices so that these files can be provided quickly to an application which requests any of these files. However slower storage devices are less expensive than high speed storage devices so equipping a storage system with slower storage devices leads to more cost efficient storage systems.
A storage system needs to mange the files that are held by the storage devices of the storage system. A storage system uses various concepts in order to decide if a file is placed on a fast storage device or on a slower storage device. For storage systems the concepts of policies by which it is decided where a file is stored, is well known. This concept was introduced by the data facility storage management system (DFSMS) which is a commercial system for IBM Corporation on zOS. The Tivoli Storage Manager which is also part of a commercial storage system from IBM Corporation provides also similar policy based storage management for backup and archives.
A policy generally refers to an attribute of a file and describes an action that is performed on the file depending on the attribute. For example, the storage system could consist of a tier 1 storage and a tier 2 storage. All old files should be placed on the tier 2 storage, which is a relative cheap storage media, while all files that are younger than a specific point in time should be placed on the tier 1 storage. A policy based on the attribute of file age could than be: ‘move all files older than 100 days from the expensive (tier 1) storage device to the cheap (tier 2) storage device’.
Another example of a policy that controls the distribution of files with various sized could be: ‘move files larger than 10 GB from the tier 1 storage device to the tier 2 storage device’. Both policies can be combined so that only files older than 100 days and larger than 10 GB are moved from one storage device to the other.
Another concept of managing the distribution of files on various storage devices is called hierarchical storage management (HSM). The Tivoli Storage Manger for space management for example employs the concept of hierarchical storage management. The Tivoli Storage Manager for space management monitors the used capacity of file system that is distributed between, for example, a tier 1 storage device and a tier 2 storage. If a high threshold value of used capacity on the tier 1 storage is reached files are moved to the tier 2 storage until the used capacity on the tier 1 storage reaches a low threshold value. This means however, that a file that could be moved due to a policy might nevertheless remain on the tier 1 storage. For example, a file that is older than 100 days could be moved according to the policy given about but resides on the tier 1 storage device as long as there is sufficient storage space available on the tier 1 storage device.
Even worse becomes the situation if the tier 1 storage is filled up to 100% of its capacity and no file is eligible to be migrated to the tier 2 storage because each file is required to remain stored on the tier 1 storage due to a policy. For example the distribution of files between the tier 1 storage and the tier 2 storage is managed by the policy that files older than 100 days should be moved to the tier 2 storage. If no file is older than 100 days, then no file is moved even if the used capacity of the tier 1 storage is 100%. The implementation of Tivoli Storage Management hierarchical storage management solves this problem by applying a score based policy. An individual score is computed for each file by using: score=file age×age factor+file size×size factor. The factors for age and size can be used to adjust the influence of age and size of a file for its eligibility criteria. Candidates for file movement are now selected by their score so files with the highest score get moved first while files with a low score reside on the expensive media. This avoids an out of space condition for a file system but it does not give the system administrator the ability anymore to define exact age or sizes of a file for becoming eligible to be moved.
There are therefore two concepts of managing a storage system. One concept triggers the migration of files based on policies while the other triggers the migration of files based on the used storage of the high speed storage devices. The two concepts can however not be combined without interference between each other. There is therefore a need for an improved method and data processing system of managing a storage system.